In the past, various types of semiconductor devices and memory devices were created to provide a semiconductor memory system which would incorporate these semiconductor memory devices. In the fabrication of semiconductor memory devices in order to produce a high density semiconductor memory array, it is important and critical to provide a semiconductor memory device that is reliable, small, relatively easy to manufacture, and, of course, cost effective.
In the fabrication of ROM (read only memory) type semiconductor memory arrays or systems, the technology has developed to the point where there are many different types of ROM type semiconductor memory arrays or systems. For example, one type of semiconductor ROM type array or system is a system where the data (either a "1" or a "0") can only be read from the ROM type semiconductor memory array or system and there is no possibility of changing the preset or selected memory storage pattern of "1"s and "0"s located therein. In other words, there is no chance of performing a write operation to change the information located in this type of ROM.
Another type of semiconductor memory ROM system or array is the use of a write-once, read- only semiconductor memory system or array wherein information can be written into the semiconductor memory ROM system only once and then this semiconductor memory ROM type system provides a preset amount of data therefrom. However, the important advantage of such a write-once, read-only semiconductor memory array is that each memory cell in the array that has not been subjected to a write operation can have its memory state changed from its prior "1" or "0" memory state to the other "0" or "1" memory state thereby providing a great deal of flexibility using such a ROM type memory array. Other semiconductor memory ROM type systems or arrays utilize an erasable technique or process wherein either a "1" or a "0" can be written into any desired memory cell of the semiconductor memory array or system and these information states can be repeatedly changed for each memory cell location in the memory system. One problem associated with these erasable type semiconductor memory arrays used as a ROM type memory is that the fabrication process is generally very complex and thereby expensive with a resultant difficulty in obtaining high yields.
This invention is directed to the write-once, read-only semiconductor memory array or system wherein information is written once only into a memory cell and once that memory cell receives a write operation of either a "1" or a "0" as preselected, the new preselected memory state that is now written into the memory cell cannot be changed to the other memory state for that particular memory cell location. Initially, write-once, read-only memory systems usually were fabricated with metal type fuses for the memory cells that would be burned out to provide an "open" or non-conducting memory state that is representative of a "1" or a "0", as desired and selected. The burning out of each metal fuse was permanent and resulted in the one time write operation for the write-once, read-only type semiconductor memory array. Each of the memory cells contained one of these metal fuses and, depending upon the coincidence of a combined write pulse directed from both a word driver and a write driver to that particular selected (for a write operation) memory cell, any metal fuse could be selected to cause it to burn out and create an open thereby providing a memory state of either a "1" or a "0" depending upon whether the burned out fuse condition was considered to be a "1" or a "0" memory state. These metal fuses that were used as a memory cell device were generally made solely of metal materials and were generally provided in a separate metal deposition type process operation. As a result, standard semiconductor processing techniques were generally not used in the formation of these prior metal fuse structures.
Another prior art write-once, read-only semiconductor memory array arrangement tried to overcome the even prior metal fuse technology by using a technique to breakdown the dielectric material (i.e. silicon dioxide) located on a surface of a semiconductor substrate, however, it was difficult to coordinate the amount of breakdown voltage needed which required consistently forming the same thickness of dielectric material.
Various other types of semiconductor devices were developed to try to provide a fuse type function where amorphous silicon was used as a material in these devices that acted as an insulator (non-conductor) before being subjected to a certain breakdown voltage that transformed this material from an insulator to a conductor. However, these prior type semiconductor devices required a number of additional processing operations, were fairly complex, produced the fuse type structures as part of a second level metalization process (which is more expensive and much harder to manufacture (lower yields) than using a single level metalization process operation). Also, these prior semiconductor fuse type structures were usually formed a relatively long distance away from a semiconductor substrate and were generally positioned above an underlying doped polysilicon layer or a metal conductor layer located above the semiconductor substrate to serve as an electrical conductor or electrode to connect to a plurality of similar semiconductor fuse type structures.
Accordingly, there was definitely a need to provide a new form of semiconductor memory fuse type device that would be compatible with semiconductor processing techniques, that would be relatively easier (and less costly) to manufacture, that would be closer to its underlying semiconductor substrate, that would use a relatively small breakdown voltage and which would permit the use of this semiconductor fuse type device in a single or one level metalization type structure for a write-once, read-only semiconductor memory array or system. It was especially important to provide such a single level metalization semiconductor fuse type device which would be less costly to manufacture and very reliable for use in a write-once, read-only semiconductor memory array or system.